This invention relates to systems for maintaining a cargo in a refrigerated condition over an extended duration by means of a finite amount of solid carbon dioxide which is not replenished during such duration.
It has long been the practice to refrigerate items in an insulated enclosure by placing solid carbon dioxide either directly into the storage area of the enclosure or into a separate compartment adjacent to the storage area. Such systems are shown, for example, in the following publications:
U.S. Pat. No. 2,508,385 PA1 U.S. Pat. No. 3,206,946 PA1 U.S. Pat. No. 3,561,226 PA1 U.S. Pat. No. 4,498,306 PA1 U.S. Pat. No. 4,502,293 PA1 U.S. Pat. No. 4,593,536 PA1 U.S. Pat. No. 4,704,876 PA1 U.S. Pat. No. 4,761,969 PA1 U.S. Pat. No. 4,766,732 PA1 U.S. Pat. No. 4,825,666 PA1 U.S. Pat. No. 4,891,954 PA1 U.S. Pat. No. 5,168,717 PA1 American Frozen Food Institute, "Cryogenic Railcar Project, Executive Summary Report," March 1985.
The foregoing systems have been especially applicable for shipment of refrigerated items by railcar where a finite amount of solid carbon dioxide is placed in a bunker at the top of the railcar prior to shipment and gradually receives heat through the bunker floor from the cargo, and through the railcar roof from the surrounding environment, which converts the solid carbon dioxide to a gas by the process of sublimation. The gas is vented from the bunker into the cargo area where it circulates to cool the cargo and then is exhausted to the atmosphere. In such systems, as exemplified by the above-listed U.S. Pat. Nos. 4,502,293, 4,593,536, 4,704,876, and 4,761,969, it has been a common practice to insulate the floor of the carbon dioxide-containing bunker to limit the heat transfer directly from the cargo to the carbon dioxide to avoid overcooling of the cargo. This, together with the heavy steel construction of the railcar which functions advantageously as a heat sink, has had the effect of extending the period during which the cargo can be maintained in a refrigerated condition without replenishing the carbon dioxide to durations of as much as 12 to 15 days, with carbon dioxide sublimation occurring over a substantially shorter period (until exhaustion of the solid carbon dioxide) followed by gradual warming of the cargo. A railcar modified and used commercially in 1991 by the present inventor, for example, was capable of maintaining adequate refrigeration of a cargo over a 12-day duration employing a carbon dioxide bunker floor which, although insulated, provided a heat transfer rate greater than 0.08 BTU per hour per square foot per degree Fahrenheit of temperature difference between the top and bottom of the bunker floor. This caused exhaustion of the solid carbon dioxide after seven to nine days, depending on the ambient temperature, followed by gradual warming of the cargo.
What has not previously been accomplished nor considered feasible is the attainment of significantly longer refrigeration durations utilizing a finite, nonreplenished amount of solid carbon dioxide, and not necessitating the heavy steel heat sink characteristics of a railcar to achieve such durations. Nevertheless there is a great need for such a low-maintenance refrigeration system for longer-duration shipments, particularly transoceanic shipments.